1. Field of the Invention
The present invention relates to an information signal recording apparatus and an information signal reproducing apparatus for recording information signals on or reproducing information signals from a recording medium in which a number of tracks are formed in parallel to each other. Particularly, the present invention pertains to tracking control for such an apparatus. Concretely, the present invention relates to tracking control employed in, for example, a VTR for reproducing video signals from slant tracks formed on a magnetic tape by means of a rotary head.
2. Description of the Related Art
Reproduction tracking methods for VTRs for household use have been proposed and put into practical use. An example of such s reproduction tracking method is the method (CTL method) in which a vertical synchronizing signal separated from a recording signal during recording is recorded in a dedicated control track formed in a longitudinal direction of a tape by means of a fixed magnetic head provided at part of a tape path. Another example of such a method is the method (4fATF method) in which recording pilot signals having four types of relatively low frequencies are recorded in turn on tracks on which main signals, such as video signals and audio signals, are to be recorded in such a manner that the pilot signals are superimposed on the main signals by means of a rotary head for recording and reproducing main signals and in which a tracking error signal (ATF error signal) is obtained by comparing the crosstalk components reproduced from the tracks adjoining the reproduction track during reproduction.
However, the CTL method requires a space for installing the fixed head, and is disadvantageous in terms of the size of the apparatus. The 4fATF method is advantageous in terms of reducing the size of the apparatus but has disadvantages in that the circuit configuration is relatively complicated because 4 types of pilot signals are required and because many analog circuit elements are thus required, and in that digital signals become less reliable when the pilot signals are superimposed thereon.
Recently, there has been a trend toward developing digital VTRs for household use exhibiting an increased image quality. Such VTRs require recording and reproduction of a relatively large amount of information. Therefore, it has been proposed to divide and record an image signal of a single field over a plurality of tracks, and a new tracking method employed in such a VTR has been examined. In this new tracking method, when a digital signal to be recorded is modulated, a recording signal is recorded in such a manner that a frequency component corresponding to a particular pilot signal is contained in a modulated signal, whereby a tracking controlling pilot signal is superimposed on a digital information signal.
Concretely, it has been proposed to superimpose the pilot signals in the manner shown in FIG. 1. In FIG. 1, reference numeral 41 denotes a magnetic tape. FIG. 1 illustrates a recording pattern on the tape 41. In this example, two types of pilot signals having frequencies f1 and f2 are used to obtain a tracking error signal. These pilot signals are superimposed on the main signals at every other track in the manner shown in FIG. 1. The pilot signals are recorded in a regular order: no pilot signal is superimposed in tracks whose head azimuth is (+), and f1 and f2 are alternately superimposed in tracks whose head azimuth is (-). In the recording pattern shown in FIG. 1, a signal of a single frame is divided and recorded in 10 tracks, and (1) through (10) denote track Nos. which record that signal of the single frame.
What reference numerals ch1, ch2, ch3, ch4 and ch1.sub.1, ch1.sub.2, ch1.sub.3, ch1.sub.4 in FIG. 1 refer to will be described later in the Detailed Description of the Preferred Embodiments.
However, the above-described new tracking method produces a tracking error signal (ATF error signal) shown in FIG. 2, and thus has the following problems.
Firstly, in addition to the normal tracking error locked position A where the level of the ATF signal becomes zero, the heads correctly trace desired tracks and desired information signals are reproduced by predetermined respective heads at an expected timing, there are positions D and E where the level of the ATF signal also becomes zero, the heads trace undesired tracks.
As is generally known, tracking control cannot quickly work nearby these positions. Such condition is called pseudo-stable condition. Therefore, it takes a long time for tracking control to make heads track the desired tracks. Such positions are called back locked positions.
In the tracking control, when the ATF error signal is plus, the tape is moved in a direction indicated by an arrow B. Consequently, when the ATF error signal is plus at position D, the tape is moved two tracks in the direction indicated by the arrow B so that it is locked at position A. When the ATF error signal is minus, similar control is performed in the direction indicated by an arrow C. However, since the tape is moved by an amount corresponding to the magnitude of the ATF error signal, it takes time for the track at position A to be moved, during which time the screen may be disturbed.
Secondly, the reproduction output level may vary due to a difference between the reproduction level of f1 and that of f2 (a difference in the reproduction level due to the reproduction frequency characteristics) or due to the performance of the tape used, thus varying the servo characteristics of the tracking control loop. In addition, the flutter of a capstan easily occurs at a cycle in which four tracks are reproduced, because the level of the reproduced pilot signals is varied at the cycle mentioned above. In the conventional 4fATF method, these problems are solved by performing automatic gain control (AGC) of a reproduced pilot signal by detecting the level of the pilot signal reproduced from a track from which the main signals are being reproduced. However, in the above-described new tracking method, since no pilot signal is recorded on a track from which the main signals are being reproduced by a head which is reproducing the main signals, the same structure as that obtained in 4fATF cannot be accomplished.
Thirdly, in the tracking control performed only by using the adjacent crosstalk obtained from a certain head, when a relative deviation in the mounting position of the respective heads is generated during the assembly of a rotary drum, the output level of the signal reproduced from the other head is reduced. Conventionally, this is overcome by improving the mounting accuracy. However, this increases production costs.